T-square



Sept. 10, 1963 G. s. DOLGORUKOV T-SQUARE Filed March 12, 1962 United States Patent 3,103,071 T-SQUA'RE Gregory S. Dolgorukov, 407 Fisher Bldg.,

' Ferndale 2, Mich. Filed Mar. 12, 1962, Ser. No. 178,868 Claims. (Cl. 33112) This invention relates to drawing instruments such as T-squares, and particularly but not exclusively to an improved T-square in which the head and the blade are secured together with the aid of an adhesion joint.

With the advent of plastic materials, particularly those available in sheet form, producing drawing instruments such as T-squares out of such material proved to be very inviting, and many such instruments including T-squares made of such material were placed on the market. However, many such instruments and particularly T-squares made wholly of plastic such as acrylic resins, which is sufiiciently hard to form a useable drafting instrument soon exhibited some undesirable characteristics, and in some cases even had to be withdrawn from the market because of excessive nicking and breakage. One of the most serious difficulties with all plastic T-squares as far as breakage was concerned with breaking of the blades at the heads. In a break of this nature, particularly where the head and the blade are secured together by an adhesion or solvent joint, the T-square cannot be easily repaired and has to be scrapped. The above difliculty has been interpreted by those skilled in the art as resulting from the excessive brittleness of the acrylic plastic, and a conclusion to the effect that such instruments as T-squares cannot be made out of such plastic if adhesion joint is used for securing the blade to the head. While the use of improved features of construction disclosed in my copending applications, exceedingly sound construction of T-squares has been provided, breakage of the blades close to the head presented a serious problem that could interfere with the introduction of such improved T-squares, particularly for school use wherein T-squares may be subjected to very rough treatment.

One of the objects of the present invention is to produce a T-square having an improved adhesion joint whereby the difficulties and disadvantages described above'a're overcome and largely eliminated without introducing other problems or increasing appreciably the costs involved.

Another object of the invention is to provide an improved T-square having an adhesive joint in which breakage of the blade at the head is reduced to a minimum and, if occurring, results primarily from extraneous causes, such as accidents in use, and not from the design of the T-square.

A further object of the invention is to provide an improved T-square having an adhesion joint, means being provided whereby localized stresses in the blade at the line of juncture with the head are eliminated, thus removing the cause of fatigue of materials and origination of cracks therein, as well as occurrence of excessive local strains when high momentary loads are imposed on the T-square, such as in its fall on a cement floor, etc.

A still further object of the invention is to provide an improved T-square specified in the preceding paragraph in which elimination of localized stresses and strains is attained either by removing the outer fibers of the material wherein such localized stresses would be the highest, or by distributing the strain over a wider area causing the mass of material to work more evenly in carrying the load, i.e. to equalize the strain more evenly throughout the mass of material, or by both of these expedients.

In one of its broad aspects the present invention relates to an improved joint between plastic parts, which joint enables the parts to withstand without failure much higher loads.

It is an added object of the present invention to provide an improved all plastic T-square having an improved joint, particularlyof the adhesion type, and having the advantages explained above, and which is rugged in construction, dependable in use, and is relatively inexpensive to manufacture.

Further objects and advantages of this invention will be apparent from the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification, wherein like reference characters designate corresponding parts in the several views.

FIG. 1 is an elevational view of my improved T-square embodying the present invention.

FIG. 2 is a view on an enlarged scale of the portion of the T-sq-uare encircled in FIG. 1.

FIG. 3 is a fragmentary sectional view taken in the direction of the arrows on the section plane passed through the line 3-3 of FIG. 1.

FIG. 4 is a view similar in part to FIG. 1 but showing a modified construction.

FIG. 5 is a fragmentary sectional view taken in the direction of the arrows on the section plane passed through the line 55 of FIG. 4.

FIG. 6 is a View similar in part to FIG. 1 but showing a still iurther modification.

FIG. 7 is a fragmentary sectional view taken in the direction of the arrows on the section plane passed through the line 77 of FIG. 6.

FIG. 8 is a view similar in part to FIG. 1 but showing a still further modification.

FIG. 9 is a fragmentary sectional view taken in the direction of the arrows on the section plane passed through the line 9-9 of FIG. 8.

FIG. 10 is a sectional view taken in the direction of the arrows on the section plane passed through the line 1010 of FIG. 8.

FIG. 11 is a view similar in part to FIG. 9 but showing a modified construction in which the upper recesses 0r bevels extend throughout the entire length of the blade.

It is to be understood that the invention is not limited in its application to the detail of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

In the drawing there is shown, by way of example, five embodiments of the present invention. The present in vention may be applied both to T-sq-uares made by profiling or cutting from cast and annealed sheets of plastic, to T-squares molded or cast to their final shape in steel molds, and to T-squares made in part by extruding.

Referring specifically to the construction of FIG. 1,

the T-square illustrated therein comprises a head made preferably of solid plastic material such as acrylic resin. To the head 10' there is secured with the aid of an adhesion joint a blade generally designated by the numeral 11 and made also of plastic material such as acrylic resin. On the underside of the blade 11 at the places of juncture of the blade 11 with the head 10, and adjacent the guiding edge 12 of the T-square, there are provided recesses or bevels 13. By virtue of the provision of said recesses or bevels 1-3, the 90 degree angle corners of the substantially rectangular area of the adhesion joint are eliminated as is best shown in FIGS. 1 and 2, wherein said area is indicated by dotting. It is desirable that recesses 13 extend inwardly of the blade 12 for a greater distance than upwardly thereof in order to relieve a suflicient area from the joint area 15 without cutting too much into the thickness of the blade 11. This condition is indicated in FIGS. 2 and 3 wherein the width of the recess 13 appears greater in FIG. 2 than its height in FIG. 3. This can be done by providing recesses 13 with the aid of a rotatable beveling cutter having an included angle of more than 90. I prefer to use acutter having an included angle of 130 or 140, thus giving a 25 or bevel, respectively. For a still better result, the sharp edge produced by the cutter is chamfered or rounded, as indicated at 19, in a separate operation, or the beveling cutter is shaped to produce such rounded corner in one operation, i.e. while producing the recess 13.

By provision of the recesses 13 the localized stresses and strains occurring in operation in the lower corners of the sides of the blade at their juncture with the head are greatly relieved, thus eliminating the occurrence of cracks due to fatigue of the material or due to the imposition of sudden loads on the blade of the T-squares, such as in a fall of the T-square on a cement floor. It

"canbe appreciated that without the provision of the recesses 13 the area 15 of the adhesion joint would have sharp corners having vertices at points 16 in FIGS. 1-3. In the use of the T-square, bending of the blade, particularly with movement of the free end of the blade upwardly, or by exerting a force on said free end sidewise, i.e. in the plane of the blade, or by a combination of such movements, as many occur in twisting the blade, the greatest concentration of stresses and strain, lacking provision of recesses 13, would occur at the sharp edge of the blade point 16, which stress may reach the point at which failure of the plastic material may occur at such point. This may result either in breakage of the blade or origination of a small crack at that point.

Such a crack may be hardly visible at the beginning. Nevertheless, origination even of a small crack in that corner would be fatal since such crack would gradually grow because of the bending and further stressing of the blade in use, first extending through the thickness of the blade material, i.e. fromthe sharp corner or edge at which it occurred to the upper edge of the blade, and thereupon progressing through the entire width of the blade to the other side thereof, finally causing the blade to break off from the head. Such breaking oif of the blade is very likely to occur when a T-square having even a very small crack falls on a hard cement floor. Cement floors are found in many drafting rooms.

In providing the recesses such as 13, the extreme outer fibers of the material at the sharp corner of the blade, which fibers would be subjected to the highest possible stress and strains, are removed. It will be understood that while under the conditions explained above such fibers would carry only a negligible portion of the total load, they would be overworked and strained beyond that strength as compared with material in other parts of the blade, and would inevitably fail under such excessive stresses. In addition to the removal of the extreme outer fibers of the material at the points of the greatest probable stress, the localized stresses and strains produced by the operational loads at such points are distributed more uniformly through the material. Particularly, the load is shifted from the point 16 to the locality of the point 17 which is on a flat surface and not in a sharp corner. On the other hand, at the point 18 the concentration of stresses would not be as great as they would be at unrelieved point 16 in a blade having no such recesses, since the bending moment at the point 18 will be reduced.

In the construction of FIGS. 4 and 5 the recesses or bevels designated therein by the numeral 20 extend toward the fixed end of the blade. By virtue of providing such bevels, the outer fibers of the material are removed, and the stresses and strains occurring in the T-square at the joint are better distributed. In such aconstruction, whatever concentration of stresses may occur at the points 20a would tend to cause failure of the joint rather than the materials of the blade at those points, thus producing the desired condition explained above. While this construction may be used under some conditions, it should be appreciated that the area of the adhesion joint in this construction is appreciably reduced by the provision of the bevels 20. In addition, in the case of using adhesion joints of the solvent type, occurrence of dry spots along the edge 21 of the blade 22 may be more difficult to prevent. Therefore, for these reasons construction illustrated in FIGS. 1-3 may be preferable.

While in the construction of FIGS. l-5 the recesses are provided in the blade, in the construction of FIGS. 6 and 7 the recesses eliminating the corners of the substantially rectangular joint area 15 are provided in the head. FIGS. 8,. 9, and 11 show a construction in which the recesses are provided both in the head and in the blade.

Referring specifically to FIG. 6, the head 25 of the T-square has recesses 26 provided therein in its guiding edge 27 underneath the side edges 28 of the blade 30. An intermediate recess 31 may also be provided between the recesses 26 in order to prevent the solvent or the adhesive agent with plastic material dissolved in it from coming out from the joint, and extending after solidifying beyond the guiding edge 26, thus interfering with the precision of the T-square. The recesses 26 and 31 may be inclined such as shown in FIG. 7 or they may have a .flat bottom and substantially vertical walls such as in the recesses of FIGS. 8 and 9. However, in either case, such recess should be provided on the top head edges and extend transversely to the guiding edge of the head in order to eliminate and preferably to round the corners of the joint area adjacent the guiding edge of the joint area, such as indicated at 29 in FIG. 6.

In the connection of FIGS. 8 and 9 the head 35 having a'guiding edge 36 is provided with a curvilinear recess generally designated by the numeral 37 and having an outline or form as indicated. By virtue of providing such a recess, the lower side 38 of the adhesion joint area 39 comes out curvilinear, particularly convex in its outline, as shown, with the corners 40 thereof merging smoothly into the sides 41 of said area. Recesses or bevels 42 similar in part to recesses 13 of the construction of FIGS. 1-3 are provided in the blade 43 on the under surface thereof. However, in the construction of FIGS. 8 and 9 these recesses are provided in such a manner that their middle portions are disposed substantially over the points 44 at which the ends of the side 38 merge with the sides 41, respectively, as is best shown in FIG. 9.

As is shown in said FIG. 9, in addition to the recesses 42 on the under surface :of the blade 43 there are also provided similar recesses 45 on the upper surface of said blade. These recesses may be either in exact vertical registry or the upper recesses or bevels 45 may be slightly offset toward the free end of the blade, as shown in FIG. 9. By virtue of providing recesses 45 localized tension stresses in the sharp edge of the blade at its juncture with the head are eliminated also when bending :of the blade,

is in the opposite direction, i.e. with the free end thereof moving downward.

The upper recesses or bevels such as 45 may be made continuous, i.e. extending throughout the entire length of the blade 46 as shown in FIG. 11 at 47. By virtue of such a construction, in addition to the function of relieving localized stresses such bevels 47 also serve an important function of decreasing the liability or" the upper edges 48 of the blade 46 to nicking. Furthermore, they decrease the thickness of the working or guiding edges of the blade (see FIG. 11) to the desired thickness as indicated at 49. I have found, for instance, that a T-square having a 24" long blade and a 9" or /2 long head shows the best characteristic if its blade is made .150-.185" thick. Otherwise, the T-square has too much Whip resulting in various problems and inconveniences. However, for the most convenient use of the T-square, its working edge should be only approximately .125" thick, which makes the above thickness of .l50-.185 undesirable. In ac cordance with the invention, I make the blades of my T- squares approximately .165".175 thick and provide the continuous bevels, such as indicated at 47 in FIG. 11, approximately .045.050" deep, thus satisfying the above contradictory and mutually exclusive requirements.

For the purposes disclosed in my co-pending applications the sharp longitudinal edges of the blades in all of the T-squares shown in the drawings may be chamfered for approximately .O20"-.030, such chamfer, for the purposes of simplifying the drawings, being shown herein only in FIG. 7 at 306.

In addition, any or all of the blades of the T-squares shown in the drawings may have their free ends finished as disclosed in my co-pending applications and may be provided at their free ends with holes, such as hole 50 shown in FIG. 10. The walls forming said hole may be rounded to produce a curvature having a diameter ap proximately equal to the thickness of the blade or coming close thereto with the resulting curved surface being polished to luster to restore the shiny surface. Such hole should be made only approximately /4" in diameter. I completely eliminate the constructions of T-squares in which such holes are made of much larger diameters such as /z" and even A2 with sharp edges. I have found that such prior constructions produce concentration of stresses in the end of the blade and invite failure thereof.

Also, the corners of the fixed end of the T-square blades are rounded, with the heavy bevel approximating 75% of the blade thickness being provided on the end edge of such fixed end. The sharp corner edges so produced are removed by provision of bevel such as indicated at 51 in FIG. 6, which make handling :of the T-squ-are more pleasant. Such bevels also have a distinctive and pleasing ornamental effect.

It shall be understood that the term adhesion joint is used herein to define joints securing connected parts throughout the entire area of their contact and to distinguish such joints from those produced by the use of connectors such as screws and the like. Therefore, said term adhesion joint includes also cohesion joints, glued joints, and other joints of the nature specified above.

Producing the joint such as illustrated in the drawings may also be effected without recessing either head or blade for eliminating the sharp corners of the joint area at the guiding edge of the head by causing the formation of such rounded corners by expedients such as preventing adhesion of the plastic materials of the head and the blade at such corners. This may be done by inserting between the head and the blade, in assembling the joint, pieces of material, such as thin metal shims, and removing them after the joint sets; or, by otherwise preventing or destroying adhesion of the head and blade materials at such corners. Another way of producing the same result is to produce the joint using a thin plastic film or piece of the desired outline for securing the adhesion rather than by using liquid solvent. Such a film may be .010"-.015

thick with rounded corners or curvilinear lower side or even completely circular in outline. It may be soaked in suitable solvent and then placed between thehead and the blade to produce the joint.

It will be seen that the present invention may also be applicable to one piece T-squares produced by molding operations permitting them to be made considerably thinner in their sections than is customary to do. The necessity of making one piece molded T-squares sutiiciently thick to prevent their bending and almost inevitable breaking up after relatively short use, makes molded T-squares bulky and heavy, requiring excessive amount of material and producing an item having the feel more of a carpenters tool than of a drafting instrument.

There is thus provided an improved T-square whereby the objects of the present invention and numerous additional advantages are attained.

I claim:

1. A T-square comprising a plastic head having a guiding edge and a plastic blade having side edges, a free end, and a fixed end; said fixed end being secured to said head by means of an adhesion joint, the area of said joint hav ing its side along the guiding edge of the head extend closer to said edge in its middle than at its ends adjacent to the sides of the blade.

2. A T-square comprising a plastic head having a guiding edge and a plastic blade having side edges, a free end, and a fixed end; said fixed end being secured to said head by means of solvent adhesion joint, the area of said joint having one of its sides extend along the guiding edge of the head but at a distance therefrom, the ends of said side at the sides of the blade being turned away from the guiding edge of the head.

3. In a T-square having a head and a plastic blade secured to said head by an adhesion joint, the longitudinal edges of the blade at the surface thereof contacting the head being beveled, with said bevel extending from the line of juncture along the guiding edge of the head toward the free end of the blade for a distance equal at least to the size of said bevel.

4. In a T-square having a head provided with a guiding edge and a plastic blade secured to said head by an adhesion joint forming a substantially rectangular area, the corners of said joint area at the guiding edge of the head being eliminated.

5. A T-square comprising two connected members, namely, a plastic head provided with a guiding edge and a plastic blade, said members being connected together by an adhesion joint forming a substantially rectangular area, the corners of said joint area adjacent the guiding edge of the head being eliminated by recessing one of said members.

6. In a T-square having a plastic head provided with a guiding edge and a plastic blade secured to said head by an adhesion joint forming a substantially rectangular area, the lower edges of the blade at the line of its juncture with the head being provided with recesses to eliminate corners in the joint area adjacent the guiding edge of the head.

7. In a T-square having a plastic head provided with a guiding edge and a plastic blade secured to said head by an adhesion joint forming a substantially rectangular area,

the upper edge of the guiding edge of the head at its juncture with the blade being provided with recesses whereby the corners of the joint area at the guiding edge of the head are eliminated.

8. The T square defined in claim 7, with an intermediate recess provided in the upper edge of the guiding edge between the other recesses, said intermediate recess extending into the upper surface of the head for a smaller distance than said other recesses.

.9. In a T-square having a plastic head provided with a guiding edge and a plastic blade secured to said head by an adhesion joint forming a substantially rectangular area, a recess provided in the upper edge of the guiding edge to produce a convex curvilinear side in said area along the guiding edge, with the ends of said curvilinear side merging smoothly into the two adjacent sides.

'10. -A T-square comprising a head having a guiding edge and a plastic blade having side edges, a free end and a fixed end; said head and said blade at its fixed end being secured together by means of an adhesion joint, the area of said joint extending within the confines of the outline of the head but further toward the guiding edge of the head in the middle of said blade than at the side edges thereof,

said blade having thickness exceeding that of its side edges, with said blade being beveled along its sides at their top surfaces fora depth equal to said excess thickness.

References Cited in the file of this patent h UNITED STATES PATENTS 903,648 Wiles 'Nov.1'0,1908 2,879,474 Beebe Feb. 24, 1959 

2. A T-SQUARE COMPRISING A PLASTIC HEAD HAVING A GUIDING EDGE AND A PLASTIC BLADE HAVING SIDE EDGES, A FREE END, AND A FIXED END; SAID FIXED END BEING SECURED TO SAID HEAD BY MEANS OF SOLVENT ADHESION JOINT, THE AREA OF SAID JOINT HAVING ONE OF ITS SIDES EXTEND ALONG THE GUIDING EDGE OF THE HEAD BUT AT A DISTANCE THEREFROM, THE ENDS OF SAID SIDE AT THE SIDES OF THE BLADE BEING TURNED AWAY FROM THE GUIDING EDGE OF THE HEAD. 